1. Field of the Invention
The invention pertains to a catalyst useful for the epoxidation of an olefin to an olefin oxide. More particularly, the invention pertains to an improved catalyst useful for the epoxidation of ethylene to ethylene oxide. The catalyst has improved selectivity in the epoxidation process.
2. Description of the Related Art
There is a continuing interest in producing improved catalysts for the epoxidation of olefins. Of particular interest are catalysts for the highly selective epoxidation of ethylene. These catalysts typically comprise a porous refractory support such as alpha alumina, which has on its surface a catalytic amount of silver and at least one promoter that helps to increase selectivity in the epoxidation process. The use of alkali metals and transition metals as promoters for silver catalysts is well known for the production of ethylene oxide by the partial oxidation of ethylene in the vapor phase. Examples of catalysts are disclosed in U.S. Pat. Nos. 4,010,155; 4,012,425; 4,123,385; 4,066,575; 4,039,561 and 4,350,616. Such highly selective catalysts contain, in addition to silver, selectivity-enhancing promoters such as rhenium, molybdenum, tungsten or nitrate- or nitrite-forming compounds, as discussed in U.S. Pat. Nos. 4,761,394 and 4,766,105. The catalyst may comprise further elements like alkali metals as described in U.S. Pat. No. 3,962,136 and U.S. Pat. No. 4,010,115.
Over the last two decades, rhenium was described as being effective in improving the selectivity of alkaline metal promoted silver-based catalyst supported by a refractory porous support. Some references in the art are U.S. Pat. Nos. 4,761,394 and 4,833,261. The further improvement of silver-based catalyst promoted with alkaline metals and rhenium by the use of sulfur, Mo, W, Cr was disclosed in U.S. Pat. Nos. 4,766,105; 4,820,675 and 4,808,738. EP 1 074 301 B1 and WO 2002/051547 disclose a catalyst containing aluminum oxide, titanium oxide and/or silicon oxide and at least one element of the first and second main group, an element of the third sub-group and an element of the eighth sub-group with bimodal pore size distribution for the dehydrogenation of C2-C16 hydrocarbons.
Beside the chemical composition of a supported silver-based epoxidation catalyst, the physical characteristics of the finished catalyst as well the support have been an integral part of catalyst development. Generally, the silver-based catalyst support shows a characteristic pore volume and pore size distribution. Furthermore, the surface area and the water absorption are well-known characteristics for such catalyst supports. It has now been found that the physical characteristics of the finished catalyst and the impact of the characteristics on the catalyst performance are more complicated than heretofore believed, especially if the catalyst is promoted with rhenium. In addition to the surface area, the pore volume and the pore size distribution, the pattern of the pore size distribution, especially the number and the specific characteristics of different modes, has now been found to have a significant positive impact on the catalyst selectivity.
It has been unexpectedly found that improved catalysts selectivity can be attained when the silver, rhenium and promoters are deposited onto a solid support having a bimodal pore distribution.